Broadly, this invention relates to a process and apparatus for exchanging heat between different-temperature objects capable of emitting and absorbing radiation. More specifically, it relates to a process and apparatus which is useful in conserving heat in systems wherein a line of workpieces are serially heated, worked, and subsequently cooled, and to a method and apparatus for subjecting a workpiece to a controlled thermal cycle wherein the workpiece is heated or cooled at a controlled rate or maintained at a given temperature.
Most industrial thermoprocessing of materials such as metals, ceramics, and glasses takes place at temperatures at or above about 1000.degree. F. At such temperatures, at least half and frequently considerably more of the heat transferred from such materials will be lost via the emission of electromagnetic radiation, predominantly in the infrared but also in visible regions of the spectrum. In general, the higher the temperature of the material in question, the greater is the contribution of such thermal radiation to the total heat loss. For example, a steel part at 1000.degree. F. in an 80.degree. F. ambient environment will have an emissivity of about 0.85 and will radiate about 6.6.times.10.sup.3 BTU/hr per square foot of surface area. Subtracting the background radiation impinging on the part, the net radiative loss amounts to about 6.45.times.10.sup.3 BTU/hr per square foot of surface area. Assuming that the natural convection of air occurs with a convective film coefficient of unity, the net convective loss from the steel part under these conditions will be approximately 920 BTU/hr per square foot. The total heat loss from the part will therefor be 920+6,455=7,375 BTU, 6,455 or 88% of which occurs via emission of radiation. Of course, as the temperature of the material is increased above 1000.degree. F., the radiant heat loss becomes an even greater fraction of the total heat loss.
Some of the consequences of the foregoing phenomenon may be appreciated in the context, for example, of a walking beam preheating furnace in a production steel mill, into which steel billets are introduced in a continuous stream at room temperature. The billets are first heated as they pass over the bed of the beam furnace to approximately 2400.degree. to 2500.degree. F. and are thereafter immediately conveyed to a forming station where mechanical working of the billet takes place. While the workpieces lose some heat at the forming station, they leave the forming station at high temperature, typically in the range of 1500.degree.-2000.degree. F. Similar practices are currently in use in certain types of ferrous forging, copper forming, stainless steel working, and other areas of materials processing, including the production of certain high performance fibers and graphite as well as metals.
With the increasing cost of energy, the concept of somehow transferring heat from the high temperature workpieces exiting the forming station to the incoming pieces at room temperature to conserve heat and decrease total energy requirements becomes increasingly attractive. This general approach to conserving heat has long been appreciated, as evidenced by the disclosures of U.S. Pat. Nos. 1,038,901, 1,286,907, 1,332,501, 1,792,423, and 2,608,740, all of which disclose the concept of conserving heat in situations similar to the foregoing by passing incoming and exiting workpieces through a tunnel-like enclosure so that heat is transferred from higher to the lower temperature workpiece. However, when the relative contribution of radiative and convective heat gain and loss is appreciated, it becomes clear that only a small fraction of the available heat energy will be exchanged when hot and cold workpieces are simply placed in proximity in an enclosure. Thus, of the total radiation emitted, only that fraction which happens to leave the surface of the high temperature object in a direction that will lead it to the cooler object will be transferred.
In addition to the foregoing, there is another area of industrial heat treating where this phenomemon is significant. Specifically, it is often required to pass certain materials such as metals (especially steels) and glass through controlled thermal programs or cycles in which the temperature of the material is held at a fixed value for a given period of time, reduced to a lower value, possibly at a controlled rate, held at the lower temperature for an additional period of time, and otherwise controllably heated, cooled, or maintained at a given temperature. Conventional methods of heat treating materials in this manner involve sequential immersions in a quenching fluid and exposures to hot furnace beds. Typically, problems of control and problems with chemical reactions occurring on the surface of the material are encountered.
Various material processing techniques employing thermal cycling of this type are now in practice, but such techniques could be applied to many other materials if a better and less expensive process and apparatus for subjecting metal or glass to such cycling were available.